1. Field of the Invention
The invention relates to a process for producing metal powder from molten metal in which a stream of molten metal leaving a nozzle element of a metallurgical vessel is broken down into droplets in an atomization chamber by gas beams and these droplets subsequently freeze (solidify) into essentially spheroidal powder grains.
The invention further relates to a device for producing metal powder from molten metal which comprises an atomization chamber into which a molten metal stream can be introduced or fed from a metallurgical vessel through a molten metal nozzle element and gas nozzle elements providing gas beams which can impinge on the molten metal stream to eventually break it down into droplets that freeze into grains, thereby yielding the metal powder.
2. Discussion of Background Information
Gas-atomized metal powders are being increasingly used in material and surface technology because of the rising quality demands on the products. The type of use determines an advantageous powder grain size and grain size distribution thereof, i.e., the respective fraction of powder grains with a specific diameter in a range of diameters. For flame spraying for surface coating of objects, for example, use of a so-called monogram powder is advantageous both from a process engineering standpoint and economically. However, in the production of parts made from metal powder using high-temperature isostatic pressing (HIP), this powder should advantageously have a high bulk density and thus have an appropriate grain size distribution.
Gas-atomized metal powders are produced essentially by causing gas, preferably inert gas or noble gas, which has a high flow speed and/or kinetic energy to impinge upon a fluid metal stream. The gas impingement causes a breakdown of the metal stream into fine droplets, which subsequently freeze to form spheroidal grains. In addition to the temperature, the viscosity and the surface tension of the fluid metal, the acceleration of the molten metal by the gas beams or the forces acting thereon are the determining factors for the size and the size distribution of the powder grains formed (Claes Tornberg in “Powder Production and Spray Forming, Advances in Powder Metallurgy & Particulate Materials—1992”, Volume 1, Metal Powder Industries Federation, Princeton, N.J., pp 137–150, “Particle Size Prediction in an Atomization System”, expressly incorporated by reference herein in its entirety).
If a free-falling metal stream is impinged upon in an atomization chamber by at least one gas beam, which can be an operationally reliable process, the achievable minimum powder grain size with respect to the main part of the fraction is limited since a high proportion of the gas beam energy gets lost in the zone between the gas nozzle and the metal stream. As a result thereof, the average grain diameter, as determined by sieve analysis (according to DIN 66165), of, e.g., high speed steel (HSS) powders produced by a corresponding process usually is about 130–150 μm, with the fraction of grains having a diameter above 1 mm accounting for about 2–5 wt-%. The tap density (the term “tap density” is the general expression for powder content after vibration of a container or capsule containing the powder) of such a powder usually ranges from 67 to 69% by volume. To increase the quality of the product, the desired grain size of the metal powder can be adjusted by screening out the coarse components; however, lower yield or reduced economy of production is associated therewith.
To improve the quality of the products made of or with metal powder and, in particular, to improve economy, it has long been an object to find a process which enables the production of a spheroidal metal powder with a high fine grain fraction and with a high yield.
If a breakdown of the comparatively dense stream of molten metal does not occur immediately, but if it is first flattened instead, the effect of the gas beam impinging on the fluid metal is intensified and finer droplets are formed which assume a spheroidal shape due to surface tension before freezing. The reduction in the diameter of the powder particles is, as previously stated, essentially dependent upon how fast the molten metal is accelerated.
Gas atomization processes for molten metals are known in which the fluid metal is broken down immediately after leaving the nozzle element of the metallurgical vessel by one or a plurality of gas beams from nozzles arranged directly at the nozzle outlet. Since, on the one hand, the gas has a high speed at the outlet and, on the other hand, quickly expands because of the effect of the high temperature and loses effect in the direction of the center of the beam, an extremely broad metal powder fraction with coarse and fine components is formed.
To avoid the aforementioned disadvantage, it has been proposed, according to U.S. Pat. No. 2,968,062, to use a device with an outwardly expanding molten metal nozzle and to design the gas feed channel concentrically around this nozzle in the shape of a cone. The gas beam generates a central underpressure which causes the molten metal to flow to the edge of the expanding outlet port, where this thin molten metal film is picked up by the gas beam and effectively broken down and accelerated. While very fine grained powders can be produced with such devices, their tendency to fail frequently and the low quantity of molten metal which can be processed thereby are disadvantageous. The disclosure of U.S. Pat. No. 2,968,062 is expressly incorporated by reference herein in its entirety.
To improve the functional reliability of the atomization device, U.S. Pat. No. 4,272,463 proposes allowing the stream of molten metal to leave the molten metal nozzle element in a free-fall and impinging on it with gas beams after a falling stretch. Despite the use of nozzles which form gas beams with supersonic speed, no acceleration of the molten metal adequate for the formation of powder grains with a advantageously small diameter could be obtained. The disclosure of U.S. Pat. No. 4,272,463 is expressly incorporated by reference herein in its entirety.
An attempt has already been made to use smaller distances between the nozzles to increase the accelerating effect of the gas beams directed at the free-falling metal stream. However, in the nozzle region, gas vortices are induced by the suction of the gas beam being discharged and/or because of the ejector effect, which gas vortices can entrain or return droplets if the distance between the nozzles and the breakdown site of the metal stream is too small. These entrained or returned droplets ultimately settle on the nozzle elements and have a destabilizing effect on the process (plugging of the nozzle elements). For these reasons, a minimum distance between nozzles must be provided which, on the other hand, unduly reduces the efficiency of the gas beam with regard to breaking down the molten metal into small droplets. For example, when a gas stream leaves a Laval nozzle at supersonic speed, its force at a distance of 30 times the nozzle diameter is reduced by approximately 50%.
From SE-AS-421758 a device for producing metal powder has become known in which two gas beams are used to break down the molten metal stream in the atomization chamber. The free-falling molten metal stream is impinged upon by a first gas beam at an angle of approximately 20°, which results in breakup and deflection of the stream, whereafter it is vertically broken down into metal droplets by a second gas stream of high intensity. While adhesion of metal droplets on the gas nozzle parts is avoided with this process, the large distance of the second nozzle from the breakdown point of the molten metal causes a broad grain size distribution with a small fraction of (desirable) fine powder. The disclosure of SE-AS-421758 is expressly incorporated by reference herein in its entirety.
A process for impingement on a vertical metal stream by a horizontal gas beam is proposed in U.S. Pat. No. 4,382,903, in which an advantageously smaller distance between nozzles is used. To prevent adhesion of metal droplets on the nozzle element, an auxiliary gas beam, aimed at an angle toward the breakdown site, is formed in the nozzle region. The breakdown of the compact molten metal stream occurs almost exclusively in the center of the horizontally directed primary gas beam, such that the yield of fine grained powder is low. The disclosure of U.S. Pat. No. 4,382,903 is expressly incorporated by reference herein in its entirety.
Another process for producing metal powder by impingement on a molten metal stream by horizontal gas beams is disclosed in International Patent Application WO 89/05197. According to this process, two flat gas beams with an essentially vertical narrow side are aligned at an acute angle to one another and the molten metal stream is introduced in the region of the collision of the beams such that first the surface zone and then the other partial zones of the metal stream are impinged upon by the gas beams. Because of the increased breakdown zone or due to the length of the distance over which the breakdown of the fluid metal occurs, the specific action of forces on the fluid metal is high; however, the energy of the gas beams is restricted by the limit of the speed of sound. A metal powder produced in this manner has a narrow grain diameter range; the fine and coarse particles are present only in small quantities, such that this powder tending toward a monogram has disadvantages for some applications because of its low bulk density. The disclosure of International Patent Application WO 89/05197 is expressly incorporated by reference herein in its entirety.
All commercial processes for producing metal powder economically in large batch sizes from molten metal and the devices which can be used therefor have in common the shortcoming that the fine powder fraction is too small and/or the grain size distribution is disadvantageous for economical further processing into high-quality products.